这句英语的语法怎么解释呢they were unaware of the fate that was to befall them

他们没有意识到将会降临到他们头上的厄运。

is to的例句：A good game is to destroy an incompetent publishers

be to do可以表示将要的意思，这里可以理解成过去将来时

烟台扬格

be to 即将
相当于that was going to befall them

他们要倒霉了

be to 即将发生的